Particle-optical apparatus comprising a fixed diaphragm for the monochromator filter

ABSTRACT

An electron microscope comprises an energy-selective filter (10) which is arranged ahead of the high-voltage field in the electron gun (2). Because the filter carries high-voltage potential and is arranged within the gun space (14) which is filled with SF 6  gas, problems arise regarding electrical and mechanical passages to the filter. Notably the centering of the filter is problematic. In order to enable suitable aperture adjustment of the filter nevertheless (for current limitation and for avoiding optical aberrations introduced into the beam by the filter), there is provided an entrance diaphragm (30) which is rigidly connected to the filter parts, notably to a pole face or to a field-defining closing piece (48a) of the filter.

The invention relates to a particle-optical apparatus, comprising aparticle source for producing a primary beam of electrically chargedparticles, which particle source comprises high-voltage means forestablishing within the particle source an accelerating high-voltagefield which is to be traversed by the particles, and a monochromatorfilter assembly which is situated substantially completely ahead of thehigh-voltage field within the source so as to select from the primarybeam a sub-beam with an energy dispersion which is less than that of theprimary beam.

An apparatus of this kind is known from U.S. Pat. No. 5,300,775.

In a particle-optical apparatus, for example an electron microscope, alow energy dispersion in the particle beam (electron beam) is generallydesirable. This is because the energy dispersion in the electron beam,in conjunction with the chromatic aberration of the imaging lens orlenses, degrades the resolution of the image of the electron microscope.A low energy dispersion is desired notably in electron microscopes inwhich electron spectroscopy is carried out, i.e. electron microscopes inwhich the energy loss of the electron beam in the sample to be studiedis determined in dependence on the location on the sample. Conclusionsregarding the composition of the sample can be drawn on the basisthereof. The contrast in the image of the sample can also be enhanced byselecting exclusively electrons with a given energy loss to participatein imaging. For these situations it is necessary to have an irradiatingelectron beam with a low energy dispersion available. This can beachieved by arranging an energy-dispersive element, also referred to asa filter assembly, in the irradiating beam. This enables energyselection by deflecting the electrons, i.e. by spatially separating theelectrons, in dependence on their energy and, if necessary, by selectingthe electrons of the desired energy.

The cited United States Patent document discloses an electron microscopewhich comprises an energy-dispersive unit (a filter assembly) for energyselection in an electron beam. As is customary in such apparatus, theelectron microscope comprises an electron gun provided with an electronsource for producing a primary electron beam, i.e. an electron beamwhich has not yet been subjected to energy selection. Moreover, the guncan be introduced into an accelerating potential (i.e. an acceleratinghigh-voltage field) in order to accelerate the electrons in the beam.The filter assembly is arranged in the gun in the vicinity of theelectron source, so that the electrons enter the filter assembly with acomparatively low energy (for example, of the order of magnitude of 3kV). This has the advantage that the filter may have a comparativelycompact construction because the dimension of a filter for an electronbeam is determined, generally speaking by the energy of the beam to befiltered. The compact construction of such a filter enables it to bebuilt into an electron microscope of an existing design, withoutextensive modification of the apparatus being required.

The described arrangement of the filter assembly ahead of thehigh-voltage field, however, also has a number of drawbacks. A firstdrawback is due to the fact that the accelerating electrode (the anode)in the electron gun of an electron microscope carries ground potential,so that the electron source carries the negative accelerating highvoltage of the order of magnitude of -300 kV. Consequently, the filterassembly also carries approximately this high voltage (since it isarranged ahead of the accelerating field), so that manipulation of thefilter component is substantially obstructed and almost impossible inpractice.

A second drawback is due to the fact that for high-voltage insulationmany electron microscopes comprise an envelope around the gun spacecontaining an insulating gas, such as sulphur fluoride (SF₆). Theaccessibility of the filter assembly is thus strongly reduced. Theformation of passages, either mechanical or electrical, through thisgas-filled space gives rise to problems as regards the gas-tightness andalso as regards the electrical insulation of the passages. (The existingelectrical connections to the electron gun are fed in via a high-voltagecable of standard design which has been taken into account for design ofthe microscope. However, this standard cable is not suitable fortransmitting electrical signals other than those for which it has beendesigned).

A third drawback is due to the fact that it would be necessary to makeholes for passages in the microscope column of an electron microscopewhich may already have been installed at a customer. This wouldnecessitate complete disassembly, involving contamination of the vacuumspace of the microscope, and also the transporting of heavy precisiontools.

Notably the alignment of the various filter components relative to oneanother and to the remainder of the electron microscope is seriouslyhampered by the above problems.

It is an object of the invention to provide a particle-optical apparatusof the kind set forth in which the filter assembly in the electron gunnecessitates only a minimum number of passages through the wall of theapparatus. To this end, the particle-optical apparatus in accordancewith the invention is characterized in that there is provided adiaphragm which is situated at the entrance side of the monochromatorfilter assembly and is rigidly connected to a part of the monochromatorfilter assembly in normal operating conditions.

In the context of the present invention the term "rigidly connected" isnot intended to mean that the entrance diaphragm cannot be detached, butthat it is not adjustable during normal operation of theparticle-optical apparatus.

As a result of these steps it is achieved that, because of the rigidconnection, the diaphragm can be accurately centered (outside theelectron microscope) with respect to the filter assembly, if necessaryby utilizing centering tools.

These steps also enable the filter assembly to transmit adequate beamcurrent (order of magnitude: 50 nA) so as to achieve suitable imaging inthe electron microscope, notably in a transmission electron microscope,by a suitable choice of the diaphragm aperture; however, on the otherhand it is ensured that this current does not become many orders ofmagnitude too large, because the energy dispersion in the beam byinteraction of the beam electrons (the so-called Boersch effect) must beprevented.

It would be feasible to achieve the desired limitation of the beamcurrent by utilizing an aperture of very small dimensions in theextraction electrode in the electron gun; however, the beam emanatingtherefrom is then so narrow that alignment of the filter assemblyrelative to the primary beam becomes highly problematic. As a result ofthe steps in accordance with the invention, a wide beam emanates fromthe extraction electrode, so that the entrance diaphragm is alwaysexposed by this beam and hence no centering problems occur.

The arrangement of the diaphragm in accordance with the invention alsooffers the advantage that the lens aberrations due to electrons whichare incident too far from the optical axis are avoided in the filter;such lens aberrations could not be reduced by beam limiting behind thefilter (i.e. the lens aberrations which cause the electrons ofundesirable direction to cross the optical axis behind the filter).

In an embodiment of the invention the filter assembly is constructed asa Wien filter. This filter is preferably provided with a permanentmagnet for generating the magnetic field of the filter.

It is thus achieved that it is not necessary to feed electricalconductors for generating the magnetic field into the space in theelectron microscope in which the filter is situated.

These and other aspects of the invention will be apparent from andelucidated with reference to the embodiments described hereinafter.

In the drawings:

FIG. 1 is a diagrammatic sectional view taken in a plane through theoptical axis of the parts of relevance of an electron microscopecomprising the filter assembly in accordance with the invention;

FIG. 2 illustrates the beam path in the filter assembly in accordancewith the invention;

FIG. 3a is a diagrammatic plan view of the filter assembly in accordancewith the invention;

FIG. 3b is a diagrammatic sectional view taken in a plane through theoptical axis of the filter assembly in accordance with the invention.

FIG. 1 is a diagrammatic sectional view taken in a plane through theoptical axis of the parts of relevance of a particle-optical apparatus,notably an electron microscope, comprising the filter assembly inaccordance with the invention. The electron microscope comprises aparticle source 2, notably an electron source, for producing a primarybeam of electrons which is not shown in this Figure. The electron source2 of the present embodiment is constructed as a field emission sourcewhich comprises an emissive tip 4, an extraction electrode 6 and alens-effect electrode 8 which forms a gun lens for controllablyconducting the electron beam from the electron gun to the othercomponents of the electron microscope. The assembly formed by the tipand said two electrodes constitutes an electrostatic lens which exerts afocusing effect on the primary electron beam produced bad the tip.

The electron source 2 also comprises high-voltage means forestablishing, in the electron source, an accelerating high-voltage fieldto be traversed by the particles. The high-voltage means comprise ahigh-voltage generator (not shown in the Figure) which can supply thetip and said electrodes with a high-voltage of the order of magnitude of-300 kV with respect to ground potential during operation of theelectron microscope. The extraction electrode then carries a voltage ofthe order of magnitude of approximately 4 kV with respect to the tip,whereas the lens- effect electrode is adjusted to a voltage of the orderof magnitude of 2 kV with respect to the tip.

The electrons ultimately emanate from the electron source via anaperture in the anode 12 which carries ground potential. The electronshave then been exposed to an accelerating potential difference ofapproximately 300 kV, corresponding to an accelerating electrostatichigh-voltage field present between the tip 4 and the anode 12 in the gunspace 14. In order to distribute said field uniformly across the space14 in the vertical direction, a number of for example, 8 disc-shapedelectrodes are arranged in said space, two of which (16a and 16b) areshown. The electrodes 16a and 16b are connected to a high voltage whichis valued between said -300 kV and ground potential and whose valueuniformly increases as a function of the distance from the tip. Thespace 14 is enclosed by a wall 20 of an insulating material such asaluminium oxide (Al₂ O₃).

For application of the high voltage to the disc-shaped electrodes 16aand 16b these electrodes are provided with high-voltage terminals 18aand 18b. The high-voltage terminals extend to the electrodes 16a and 16bthrough the wall 20; they are situated in a space 22 which is filledwith gaseous sulphur fluoride (SF₆) for the purpose of high-voltageinsulation. The space 22 itself is enclosed by a gastight wall 24.

In the space 14 a monochromator filter assembly 10 is arrangedunderneath the lens-effect electrode and above the electrodes 16a and16b in order to achieve a uniform high voltage distribution; the opticalaxis 26 of the filter assembly coincides as well as possible with theoptical axis of the electron microscope. This filter assembly isintended to select from the primary electron beam emanating from the tipa sub-beam with an energy dispersion which is less than that of theprimary beam. The construction and the properties of this filterassembly will be described in detail with reference to the otherFigures.

FIG. 2 illustrates the beam path in the filter assembly 10 in accordancewith the invention. In this Figure the filter assembly 10 isdiagrammatically represented by an entrance diaphragm 30 whose aperture32 is shown in exaggerated form for the sake of clarity. The filterassembly 10 is constructed as a known Wien filter, consisting of a polesystem for generating a magnetic field and an electrode system forgenerating an electrostatic field. The magnetic field is generated, forexample by a set of magnet poles 34, so that the field lines of themagnetic field extend in the plane of drawing. The electrostatic fieldextends perpendicularly to the plane of drawing and is produced by twoflat electrodes (not shown in the Figure) which extend parallel to theplane of drawing.

In FIG. 2 the primary beam 31 is limited by the aperture of theextraction electrode 6. If the limiting aperture is large, the beamcurrent in the Wien filter will be so large that in the focusing rangeof the filter a strong interaction occurs between the electrons in thebeam, the so-called Boersch effect, so that energy dispersion occurs inthe filter. In this situation energy dispersion is undesirable and,therefore, the current in the Wien filter must be limited to such anextent that (noticeable) energy dispersion no longer occurs. A suitablevalue for the beam current in a practical set-up is, for example 50 nA.To this end there is provided an entrance diaphragm 30 having anaperture which is substantially smaller than the aperture 32 shown inFIG. 2. The aperture 32 has a diameter of, for example 100 μm, whereasthe aperture in the extraction electrode 6 has a diameter of, forexample 400 μm. The diaphragm 30 is rigidly connected to the magneticpole shoes 34; it is alternatively possible to connect said diaphragmrigidly to a closing piece for closing (defining) the magnetic field, ifpresent.

FIG. 3a is a diagrammatic plan view of the filter assembly 10 inaccordance with the invention and FIG. 3b is a sectional view thereof,taken in a plane through the optical axis 26. The magnetic field in thespace 46 is generated by means of a magnetic circuit, consisting of apermanent magnetic part 36 which is in contact with an iron circuit 38,40, 42. The latter circuit is made of a material having a high magneticpermeability, for example iron, and serves to conduct the magnetic fluxfrom the permanent magnet 36 to the space 46. At the magnetic side thespace 46 is bounded by the pole shoes 34. The electric field isgenerated by the electrodes 44 which have been omitted in FIG. 3b forthe sake of clarity. Because of the use of the permanent magnet 36 forgenerating the magnetic field, electric supply leads as would berequired for excitation coils are no longer necessary. FIG. 3b showsmagnetic closing pieces 48a and 48b. If desired, the entrance diaphragm30 can be rigidly connected, via a holder 50, to the closing piece 48aat the entrance of the filter assembly 10.

We claim:
 1. A particle-optical apparatus, comprising a particle source(2) for producing a primary beam (31) of electrically charged particles,which particle source comprises high-voltage means (16a, 16b) forestablishing within the particle source an accelerating high-voltagefield which is to be traversed by the particles, and a monochromatorfilter assembly (10) which is situated substantially completely ahead ofthe high-voltage field within the source so as to select from theprimary beam a sub-beam with an energy dispersion which is less thanthat of the primary beam, characterized in that there is provided adiaphragm (30) which is situated at the entrance side of themonochromator filter assembly (10) and is rigidly connected to a part(48a) of the monochromator filter assembly in normal operatingconditions.
 2. A particle-optical apparatus as claimed in claim 1, inwhich the monochromator filter assembly (10) comprises a Wien filter. 3.A particle-optical apparatus as claimed in claim 2, in which the filterassembly (10) comprises a permanent magnet (36) for generating themagnetic field of the filter.
 4. A particle-optical apparatus as claimedin claim 3, characterized in that the diaphragm (30) is connected to apole face or a field-defining closing piece (48a) of the magneticcircuit of the Wien filter.
 5. A particle-optical apparatus as claimedin claim 2, characterized in that the diaphragm (30) is connected to apole face or a field-defining closing piece (48a) of the magneticcircuit of the Wien filter.